Electron tube protective system



Oct. 21, 1952 M. v. HOOVER 2,615,147

ELECTRON TUBE PROTECTIVE SYSTEM Filed April lo, 1951 ATTORNEY PatentedOct. 21, 1952 ELEcTRoN TUBE PROTECTIVE SYSTEM Merle Vincent Hoover,Mountville, Pa., assignor to Radio Corporation of America., acorporation of Delaware Application April 10, 1951, Serial No. 220,196

This invention relates to improvements in electron tube protectivesystems, and particularly to systems for protecting electron tubes fromabnormal localized currents, the p-resent invention being, in part, adivision of the c opending application of W. N. Parker, H. C. VanceandrM. V. Hoover, Serial Number 148,688, led March 9, 1950, now PatentNo. 2,575,232, and assigned t the `assignee of the present invention.

In a copending application of L.k P. Garner, Serial Number 148,723,`iled March 9, 19.50, now Patent No. 2,571,027, and assigned to thesameassignee as the present invention, there is described and claimed anextremely fast acting system for protecting a high vpower oscillator oramplifier tube against a high current fault, such as an'internal arc oran internal short, by short-A circuiting the power supplyof the tube.,The short-circuiting device preferably comprisesA a normallynon-conductivev electron tube `containing suiiicient gas or vapor sothat, when con;- ducting current, the voltage drop thereacross will havea low, substantially constant value characteristic of a gas or vapordischarge (such a tube being referred to hereinafter as a gastube). Theinitiation of current ilow (hereinafter designated iiring) in the gastube is arranged to occur only in response to the occurrence of a faultin the protected tube. It is one' of the objects of the presentinvention to provide an improved fault-detecting arrangement for suchprotective systems.

In the above-'mentioned Garner system, the gas tube ring voltage isderived from the current increase which accompanies a high current faultin the protected tube. While this fault detection arrangement isbasically sound, it does not provide maximum possible sensitivity underall conditions of operation. For example, ii a high power amplifier tubeis being operated Class C, sothat tube current only flows during a partof each cycle of alternating voltage, the gas tube must be adjusted sothat it'will not re until the protected tube current exceeds the normalpeak value. This makes it possible for a fault to reach seriousvproportions before the protective system responds.

Accordingly, it is a further object of theeinvenftion to provide animproved fault detection circuit which is adapted to dierentiatebetween-a high current fault in the protected tube and othernon-injurious changes in system operation.

In the above-noted copending application of W. N. Parker, H. C. Vanceand M. V. Hoover, there is described a fault detecting circuit whichdepends for its operation on the unbalance be- 17 Claims. (Cl. S15-#91)tween input and output power Awhich accompanies a high current fault inthe protected tube. Taking tube output voltage as substantiallyrepresentative of output power, and tube input current as substantiallyrepresentative of input power, the fault detecting circuit is arrangedto nre the protecting gas tube upon simultaneous Opposite changesoccurring in these two parameters, thereby insuring rapid anddependable` operation of the protective system. That is, an increase ininput power, accompanied by a decrease in output power, will causefiring of the protecting tube. Of course, achange in only one of theseparameters can cause slower ring.

In the vspecific fault detection circuit described in said copendingParker et al. application, a resistor-capacitor network isprovided toeliminate pulsating components from the voltages repre-e senting theprotected tube input and output power. The purpose of thisresistor-capacitor network is to improve system sensitivity byeliminating voltage fluctuations which otherwise would haveto beaccounted for in setting the response f level of the fault detector.However, this network also has the eiiect of increasing the responsetime of the system. That is, if the network is to be effective, the timeconstant thereof must be of the order of one half the period of thealternating voltage being amplied by the protected tube. Hence, thevoltage across the resistor-capacitor network cannot change in aninterval less than this half period. If the voltage being amplied by theprotected tube is sufficiently high in frequency, Vthis characteristicof the detection cir-' cuit will not be particularly objectionable.However, when relatively low -frequencies are involved, it is possiblefor a fault to develop beyond permissible limits before the protectivesystem will function.

It is, accordingly, a further object of the present invention to providean extremely fast-acting fault detection circuit for a tube protectivesystem. v

In accordance with the invention, the foregoing and other relatedobjects and advantages are attained in a fault detecting circuit whereinboth the voltage proportional to the protected tube input power and thevoltage proportional to tube output power are pulsating in form. Inaccordance with one embodiment of the invention, these pulsatingvoltages are of opposite polarity and are applied as separate controlvoltages to a trigger circuit which will respond to simultaneous opposite changes of the pulsating `control voltages. In-

other embodiments of the invention, pulsating control voltages of thesame polarity are utilized with novel forms of trigger circuits toobtain a voltage pulse for triggering a protecting gas tube.

A more complete understanding of the invention can be had by referenceto the following description of illustrative embodiments thereof, whenconsidered in connection with the accompanying drawing, in which Fig. 1is a circuit diagram of an electron tube protective system arranged inaccordance with the invention, and

Figs. 2 and 3 are circuit diagrams ofv modified forms of fault detectingcircuits for protective systems arranged in accordance withY theinvention.

Referring to Fig. 1 of the drawing, there is shown a triode electrontube I connected as an alternating voltage amplier, as, for example, inthe output stage of a conventional radio transmitter. It will beunderstood that the tube I8 could as well be connected as an oscillatorby providing suitable feedback connections between the anode and controlgrid circuits. Also, the protection of a tube connected as a modulatoror as a buffer is deemed to be within the scope of the invention.

The tube I0 is arranged to amplify alternating voltages applied to thecontrol grid I2 thereof through a coupling capacitor I4. The usual gridreturn resistor I6, and a bias voltage source shown as a battery I1, areconnected between the control grid I2 and the grounded end of a resistorI9 which is connected between ground and the cathode I8 of the tube I0.The tube anode 20 is connected to a resonant tank circuit 22 from whichampliiied alternating voltage can be supplied to any suitable load (notshown) such as an antenna, an additional amplii'lenor the like.

The tube anode 20 is connected through a resistor 2I and a choke coil 23to the positive terminal 24 of a unidirectional voltage supply source26.

In the case of a high power radio transmitter, the voltage applied tothe tubev anode 20 from the power supply 26 may be of the order ofseveral thousand volts. As long as the tube I8 is functioning properly,the power dissipated at the tube anode 20 will be within safe limits.However, if an arc should form between the tube anode 28 and cathode I8through stray residual gas in the tube I8, the tube current will risefar beyond its rated value, and this excessive current ordinarily williiow between localized spots on the anode and cathode. Under theseconditions, one or both of the tube electrodes I8, 20 may melt within afew hundred microseconds. Even if the electrodes do not meltappreciably, enough gas may be evolved to tend to reduce electronemission as well as to enhance the possibility of subsequent arcs. y

In accordance with the aforementioned Garner application, protection forthe tube I8 is provided by a normally non-conductive shortcircuitingdevice comprising an ignitron type gas tube 33 shunted across the powersupply 25. The main anode 34 of the ignitron 33 is connected directly tothe positive power supply terminal 24. The ignitron dividing anode 35,the auxiliary anode 38 and the grid 40 may be energized from the same ora separate power supply, as desired. An igniter electrode 42 isconnected to receive ring voltage from a fault detecting circuit, asdescribed hereinafter, and the mercury pool cathode 43 is connected toground.

In accordance with the usual principle of operation of the ignitron typetube, the tube 33 will be non-conductive until a positive voltage isapplied to the igniter electrode 42. Thereupon, a small arc will formbetween the igniter 42 and the cathode pool 43, and this small arc willspread to the anodes 34, 35,- 38,- substantially instantaneously.Although the, ignitron 33 is adapted to withstand very high voltagesprior to firing, the voltage drop thereacross after ring is very lowsayof the order of 15 or 2-3 volts. By yarranging the ignitron 33 to nre inresponse to the occurrence of a high current fault in the tube I Il, itcan be seen that the ignitron effectively will short-circuit the powersupply 26 if such a fault occurs, thereby diverting the current whichotherwise would soon ruin the protected tube.

In accordance with the present invention one section of the faultdetection circuit shown in Fig. 1 comprises means 45 for generatingv apulsating voltage proportional to the protected tube output voltage. Theother section of the circuit comprises the resistor I9, which isconnected .in the protected tube cathode circuit to comprise generatingmeans for a pulsating voltage proportional to the protected tube current(i. e. input power).

The-network 45 comprises a half wave rectifier 41, connected to the tubeoutput circuit 22 through a coupling capacitor 49 to generate a negativepulse of voltage across a potentiometer 5I during each positive halfcycle of voltagefat the grid I2 of the tube I0. A resistor 53 com pletesthe circuit forthe rectifier 41. Y

A gasy tetrode tube 58 has a control grid 58 which is connected througha biasing battery 62 to the protected tube cathode I8, and a screen grid64 which is connected to the potentiometer tap 5Ia and serves as asecond control .grid in the tube 58. The gas tetrode anode 6B isconnected to a half wave rectier power supply 39, and the cathode `t8 ofthe gas tetrode 58 is connected to ground through a load resistor 10.

As long as the protected tube vlil is operating normally, the tendencyfor the tube 58 to re in response to positive voltage impulses appliedto the control grid BIlthereof will be overcome by the negative voltageimpulses on the screen grid y84. However, if a fault should develop inthe protected tube I0, the positive voltage pulses at the control gridof the gas tetrode 58 will increase in amplitude, While the negativevoltage impulses applied to the screen grid 64 will decrease inamplitude, allowing the tetrode 58 to fire very quickly after a faultbegins to develop. When the gas tube 58 fires, a positive voltage pulsewill be developed across the load resistor 10, and this positive pulsecan be utilized to nre.

the ignitron 33.

A second gas tetrode 12 is provided for coupling4 the fault detectingcircuit just described to the ignitron 33 to re the ignitron upon theoccurrence of a fault in the protected tube I0. The cathode 14 of thesecond tetrode 12 is connectedk to the igniter electrode 42 in theignitron 33, and the anode 16 of the tetrode 12 is connected to theauxiliary power supply 39. The screen grid 18 in the second tetrode 12is connected to a source of negative voltage, shown as a'battery 80, tonormally prevent conduction in the tetrode 12. The control grid 82 ofthe tetrode 12 is connected to the vload resistor 1I] of the rst gastetrode 58 through a coupling diode 84. As shown, the diode 5 84 maycomprisetwo or more sections to provide coupling between the ring tube12 andv additional amplier tubes II! (not shown) which are energizedfrom the power supply 26. Thus, upon the occurrence of a fault in any ofthe protected 'tubes I0, the gas tetrode 12 will conduct and fire theignitron 33 to short-circuit the power supply 26. f While the couplingdiode 84 is not essential, it is deemed preferable in order to decouplethe cathode circuits of the protected tubes.

The auxiliary power supply 39 preferably isv of the half wave rectifiertype, as indicated, so that the gasftetrodes 58, 12 will deenergizereadily after firing.

In Fig. 2 of the drawing there is shown another `embodiment of theinvention wherein a diierent type of.trigger pulse generator isutilized, and

pulse-pickup arrangement comprising the coil 25 can be used toadvantage, provided the pulses induced in the coil .25 are in the propertimephase with respect to the pulses representing output power.

In place of the gas tetrode tube 58 of Figli' there 'is provided in thecircuit of Fig. 2 a cathode ray tube 28 having the usual electron gun39, a pair ofl deiiecting. electrodes 3|, 32and a fluorescent screen 3E.Other details of thecathode ray tubeA 28as well as the energizingcircuit therefor, have not been shown'since all' are well known per seand are not essential Ato an understanding of the invention. 1 Y

Adjacent the screen end of the cathode ray tube .23, there is provided aphotocel1`54 having antanode 55 connected to an' energizing voltageVsource shown as abatter'y y56. `The photo-cathode"v If a fault occurs inthe tube I0, the current iiowing thereto will rise abruptly, causing arelatively large positive pulse to be developed across the inductor 25.At the same time, the oscillatory voltage coupled into the resonantnetwork 48 will decreasecausing a decrease in the amplitude of thevoltage pulses developed across the potentiometer 5I. As a. result, thecathode ray beam will be attracted downwardly in the tube 28. and willimpinge on the lower portionvof the screen 36, say at point B. Thiswillallow the photocell 54 to conduct current, developing` a i positivepulse of voltage across the resistor 59.

51 Yof the cell 54 is returned to the voltage' source 5B'throu'gh aresistor 59.

One of the deecting plates 3| of ltheftube 28` is connected to the pulsepick upi'ndu'ctor25.y They other plate 321s connected to the tap 5|a ofa potentiometer 5I in the cathode circuit of a rectier 41.

positive voltagey pulses developed in the VinductorV 25 by pulsatingcurrent flow through the tube I II` maytend to attract the electron beamin the tube 28 downwardly, but thistendency will be offset by positivepulses developed across the poten-` tiometer 5| by rectification of theoscillatory voltage` coupled .into the .resonant circuit 48.`

Therefore, the beam in the tube 28 *will,tend to sta-y at or near thecenter of thescreen '36 (at point A).

current. If necessary, a light shield 52. can be placed adjacent to thephotocell 54 to preventany light from the screen 36 impingingl on thecathode 51 as long as the` cathode ray `beam is approximately centered.i v

In the anodecircuit of the recti-` iler 41, there isr provided aresonant circuit 48 l As long as this condition obtains; the photocell54 will notconduct any appreciablel This positive voltage pulse then canbe utilized to trigger a short-circuiting device 33 to divert power fromthe protected tube I0 in the manner already described in connection withFig. 1.

lIt will be understood that the circuit of Fig. 2 could be arranged withthe resistor I9 in the cathode circuit of tube I0 in place of theinductor 25. Similarly-.the inductor 25 could be used as a pick-upelement in a non-differential fault detecting system such as thatdisclosed in the above-mentioned (zo-pending Garner et al. application.i

In Fig. 3, a further form of fault detecting and pulse generating meansis illustrated. vIn Fig. 3, there is shown only the tube I0 and suchportions of the circuit thereof as are necessary to depict theembodiment of the invention presently being described. l

In'this case, the pulse generating means comprises a so-called neon glowtube 50 having a pair of electrodes 52 connected to a cathode loadresistor I9 for the tube I9 and a potentiometer 5| in the cathodecircuit of a half wave rectifier 41.- The circuit of the half waverectifier 41 is the same as that shown in Fig.2. I

, Avphotocell 54 isplaced adjacent tothe neon tube 58 and connected in acircuit identical withl that shown for the photocellA 54 in Fig. 2.

As longas the tube I9 in Fig. 3 is functioning properly, Voltage pulsesgenerated by tubecurrentow `through the resistor I9 .will be offsetby'voltage pulses developed acrossrthe potentiometer 5I, so that equalvoltages will be applied to the neon tube electrodes 52. Thus, the neontube will not ignite, and the photocell 54 will conduct substantially nocurrent. If a fault occurs in'fthe protected tube I9.. the'voltage-developed across the resistor` I9 will increase rapidly, whilethe voltage across-the potentiometer 5| will drop rapidly. As a result,sufiicienty voltage will be developed between the neon tubev electrodes52 to ,i generate a pulse of light from the neon tube.

This, in turn, will allow a pulse of current to flow throughthephotocell 54,A thereby to generate a' voltage pulse across theresistor 59. This. pulse developed across the resist/or 59 can then `beutilized to trigger the short-circuiting device in the manner alreadydescribed. i

It should be noted that a fault detector such as the tube 28 of Fig. 2or the tube 5D of Fig. 3. could be used with unidirectional faultvoltages' such as are developed in the system of theabovementionedParker et al. application.-

From the foregoing, it can belseen that th present invention provides anextremely fast acting fault detecting arrangement for electron tubelprotective systems, and one that is capable of execution in a variety ofsimple and eiiicient ciry cuits.

What is claimed is: v l. A fault-detecting circuit for an electron tubeconnected in a network to amplify'alternatingi.f

voltages, said circuit comprising. a rstfvoltagegenerating*meanscoupledto said network for generatingA aiirst pulsatingvoltage in response to-pulsating'ciurent flow through vsaid tube, asecond voltage generating means coupled to said tube to generateafsecond-pulsating voltage yproportional in amplitude to'the alternatingoutput voltageof said tubefa pulse 4generator circuit forgenerating.a-f-ault-indicating voltage pulse onlyfin Aresr'ionse toavcombination of pulses of predetermined amplitude' from saidv first andf second generating vvmeans, and pulse-transfer circuitsconnecting eachsaid generator to said pulse generator circuit. y

2; vA fault-detectingcircuitas defined in claim l wherein said firstmeans comprises a` first resistor connected in seri'es'with said tubeand Ysaid second means comprises ahalf wave rectier network connected to'rectify said alternating output voltage of said tube.3.'A'fault-detecting'circuit as defined in claim 1lwherein said firstmeans comprisesv a-'pickup coil inductively coupled to said network andsaid second means comprises a half wave rectifier network connected torectify said alternating out# put voltage of said tube. 1

4J fault-detectingcircuit foran electron tube protective system of thetype wherein a normally?rnonconductive .short-circuiting; -device isshunted across the 'supply voltage source for'an electron tube connectedto amplify lalternating voltages, said 'circuit comprisingl a firstvoltage generating means in circuit-with said tube for generatinga'first pulsating voltage in response to pulsating current flow throughsaid tube, a second voltage generating means coupled to receive'alternating voltage'fromsaid tube to generate a second pulsating:voltage proportional in amplitude to the alternating output voltage ofsaid tube, means connected to said first and said second generatingmeans to generate a faultindicating voltage pulse only in response to acombination of pulses of predetermined amplitude from said rst andsecond generating means, and means-coupled to said last named means toinitiate conduction in said short-circuiting device in response to saidfault-indicating voltage pulse.

5. A circuit as defined in claim 4 wherein said last named meanscomprises a cathode ray tube having deflecting electrodes connected tosaid rst and second generating means, and a photocell arranged to passcurrent in response to predetermined deflection of the cathode ray beamin said cathode ray tube.

' 6. A circuit as defined in claim 4 wherein said last named meanscomprises a neon glow tube having a pair of electrodes connected to saidfirst and second generating means, and a photocell arranged to passcurrent in response to energ-ization of said glow tube.

7 A system for protecting an electron tube connected in a circuit toamplify alternating Voltages, said circuit including a unidirectionalvoltage source connected to furnish anode voltage to ksaid tube, saidsystem comprising a normally non-conductive gas tube shunting saidvoltage source to short-circuit said source in response to a voltage ofpredetermined magnitude applied to a ring control electrode of said gastube, a resistor connected in the cathode circuit of said tube, means incircuit with said tube for generating Voltage pulses across saidresistor in response to pulsating current flow through said tube, meanscoupled to said tube to generate voltage pulsesproportionalin amplitudeto the alternatingoutput voltage of said tube,'and 'means couipled` tosaid gas tube and responsive to said pulsating voltages to provide tosaid gas tubefa fault-indicating voltage. i

8. In a fault-detecting circuit forl an 'electron tube'. connected :toamplify alternating voltages, in combination,"a first resistor in serieswithsaid tube for generating voltage pulses in response/to pulsatingcurrent flow through said tube,'a network comprising a half waverectifier and a'second resistor connected to said tube to Agenerateacross said second resistor voltage-pulses from alternating voltagereceived from said tube, a gas tube having'rst and second controlelectrodes, and connections from said resistors to s'aidcontrolelectrodes. f

9;.In a faultdetecting circuit for Iany electron tube connected' toamplify alternating voltages, incombination, afresistor in circuit withsaid tubefor vgenerating a first pulsating unidirectional voltage inresponse to current flow through said tube, a rectifier connected tosaid tube to generate a second pulsating unidirectional voltage fromalternating voltage received from said tube, a gas tubevhaving first andsecond firing control electrodes connected one to said resistor and oneto said rectifier, and abias voltage source connected to one ofsaidcontrol `grids normally to prevent current fiow in said gas tube.

'10.,In a protectivesystem for an electron tube connectedA to amplifyalternating voltages and supplied with unidirectional operating voltagefrom a high voltage source, in combination,: an ignitron gas tube havingan anode, afmercury pool cathode, and an igniter electrode, acircuitconnecting said'ig-nitron gas tubel in parallel with said voltagesource,` said electron tub'ehaving a cathode, aresistor'conneetedbetween the cathode of said electron tube Aand said voltage source todevelop acrosssaid resistor first voltage pulses proportional inamplitude to current vpulsesilowing-through saidV electronvtube, arectifier net- Work coupled to said electron tubeto generate secondvoltage pulses. proportional in Iamplitude to the alternatingoutputvoltage kof said electron tube, andmeans coupling said resistor andysaid rectifier network to saidigniter electrode to lfire said ignitrontube in response to vaxpredetermined combination of said'flrst 'andsaid'second pulse voltages.

l11.lipparatus as :defined in clairn 9. wherein said coupling meanscomprises a gastube having rst and second controlv grids; one. of saidoontrol grids being lconnected to said resistor and t-he other y, of,saidv control 'grids being 4connected to said rectifier network.

12. Apparatus as dened in claim 1l including, as a source of operatingvoltagefor ysaid gas tube, a half Wave rectifier power supply whereby topermit deenergizing said gas tube after firing thereof. l

13. In a system forrprotecting an electron tubev connected in a circuitfor amplifying alternating voltages and including `a runidirectionalvvoltage source connected to furnish anode voltage to said tube andanormally non-conductive gas tube shunting 'said voltage source toshort-circuit said source in response to lan impulse applied to a firingcontrol electrode of saidgas tube, in combination, a grid-controlled gastube having an anode, a cathode, and first and second control grids, abias voltage source connected to one of said control grids normally toprevent current owin said grid-controlled' gas tube, first means coupledto said rst controlgrid and to said electron tube circuit for generatingand applying to said rst control grid a voltage impulse of one polarityduring half of each cycle of alternating Voltage amplied by saidelectron tube, second means coupled to said second control grid and tosaid electron tube circuit for generating and applying to said secondcontrol grid a voltage impulse of polarity opposite to said one polarityduring each said half cycle, and means coupling said grid-controlled gastube to said iiring control electrode to re said shunting gas tube inrespense to current flow in said grid controlled gas tube.

14. In an electron tube protective system of the type wherein a normallynon-conductive shortcircuiting device is shunted across the supplyvoltage source for an electron tube having an anode and a cathodeconnected to said source, a fault-indicating-voltage generatorcomprising an inductor inductively coupled to said connection betweensaid cathode and. said source to generate a voltage pulse in respone toa change in the current through said connection, and means responsive tosaid voltage pulse to render said normally non-conductive deviceconductive.

15. In an electron tube protective system of the type wherein a normallynon-conductive short-circuiting device is connected in shunt with thesupply voltage source for an electron tube, and wherein means areprovided for generating separate voltages representative respectively ofthe input and output power flowing to and from said tube, theimprovement which comprises a photocell, a pulse generating circuitincluding said photocell for generating alvoltage pulse upon impingementof a light pulseon said photocell, means coupling said pulse generatingmeans to said short-circuiting device to render said device conductivein response to a pulse from said circuit, and means responsive only to apredetermined combination of said separate voltages to supply a lightpulse to said photocell.

16. A system as defined in claim 15 wherein said last named meanscomprises a cathode ray tube having deflecting electrodes connected tosaid voltage generating means. y

17. A system as dened in claim 15 wherein said last named meanscomprises a neon glow tube having a pair of electrodes connected to saidvoltage generating means.

MERLE VINCENT HOOVER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,571,027 Garner Oct. 9, 19512,575,232 Parker et al. Nov. 13, 1951

